Voice Coding - ZyXEL Communications VMG8924-B10D User Manual

The following table shows the SIP call progression.
Table 110 SIP Call Progression
UA 1
Invite
ACK
RTP
200 OK
User Agent 1 sends a SIP INVITE request to Proxy 1. This message is an invitation to User Agent 2 to
1
participate in a SIP telephone call. Proxy 1 sends a response indicating that it is trying to complete the
request.
Proxy 1 sends a SIP INVITE request to Proxy 2. Proxy 2 sends a response indicating that it is trying to
2
complete the request.
Proxy 2 sends a SIP INVITE request to User Agent 2.
3
User Agent 2 sends a response back to Proxy 2 indicating that the phone is ringing. The response is
4
relayed back to User Agent 1 via Proxy 1.
User Agent 2 sends an OK response to Proxy 2 after the call is answered. This is also relayed back to User
5
Agent 1 via Proxy 1.
User Agent 1 and User Agent 2 exchange RTP packets containing voice data directly, without involving
6
the proxies.
When User Agent 2 hangs up, he sends a BYE request.
7
User Agent 1 replies with an OK response confirming receipt of the BYE request, and the call is
8
terminated.

Voice Coding

A codec (coder/decoder) codes analog voice signals into digital signals and decodes the digital
signals back into analog voice signals. The VMG supports the following codecs.
• G.711 is a Pulse Code Modulation (PCM) waveform codec. PCM measures analog signal amplitudes
at regular time intervals and converts them into digital samples. G.711 provides very good sound
quality but requires 64 kbps of bandwidth.
Chapter 22 Voice
PROXY 1
Invite
100 Trying
180 Ringing
200 OK
VMG8924-B10D/B30D User's Guide
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PROXY 2
Invite
100 Trying
180 Ringing
200 OK
UA 2
180 Ringing
200 OK
RTP
BYE